In order to observe the optical appearance of the surface of a sample, the surface can be viewed from a certain direction, whereby a light beam is directed to the surface from another direction. Thereby, a variety of information about the surface can be obtained, depending on the direction, intensity and color of the incoming and outgoing light and on the direction of viewing towards the surface. The observed appearance may include the texture and/or relief or protrusions or projections of the surface such as hair on the skin and, in case the material of the surface itself is more or less translucent, the texture and/or color and/or morphology underneath the surface, i.e. the sub-surface. The observation can be recorded and/or analyzed.
It should be noted that the expression appearance is used in this description for each combination of aspects and/or properties of the surface of the sample and the perception of the observer of it, including the from and relief of the surface, the color of the surface, the light reflecting and light absorbing properties of the surface, etc. Observing is a general expression, it may include inspecting and/or recording and/or analyzing of the appearance of the surface.
A non-contact and non-perturbing monitoring technique is useful in many areas of technology to determine surface and/or sub-surface morphology. Furthermore, the type and density of material defects or other features, which have a geometric shape or optical perturbation, can be characterized using this technique. Another use of this technique is the analysis of the characteristics and condition of human skin.
In particular when details of the morphology are to be analyzed, it is desired to make observations of the appearance of the surface from different directions, whereby the light source also may illuminate the surface from one or more predetermined directions (different angles with respect to the plane of the surface).
In particular when the surface of a relative large object has to be observed, for example a piece of the skin of a human body, it is not possible to place the sample inside the apparatus. In that case, the apparatus should be placed on or against the sample or a part of the sample, whereby the location of the surface to be observed is at the outer side of the apparatus.
An optical measurement device for measuring an optical appearance of a surface of a sample, in particular the human skin, is disclosed in WO 2007/072403. The disclosed device comprises an illumination device for illuminating the surface with an illumination beam and a detection device for detecting the response of the sample to the illumination beam.
In the known optical measurement device a base plate is placed on a part of the sample, for example a part of the human skin, thereby touching and applying a pressure to the human skin. The part of the deformable skin, which adjoins the region on which the pressure is applied, will tend to bulge. This causes spurious reflections from the surface of the skin and causes the surface of the skin to be outside the depth of focus of the detection device which reduces the quality of the observance of the optical appearance of the surface of the sample. This disturbs the observance of appearance of the surface of the skin, because a camera, which is used to analyze the response of the human skin to the illumination beam, will produce a blurred and out-of-focus image. Furthermore, the applied pressure on the skin alters the appearance of the skin. Blood vessels are at least partly closed off, thus causing blood to drain from the skin locally and accumulate on other places, which causes red or white discolorizations of the skin.